L-Cysteine and its derivatives are used in the pharmaceuticals sector, in cosmetics and in the foodstuffs industry. For those applications, L-cysteine is typically produced in pure form.
L-Cysteine is traditionally obtained by extraction from materials containing keratin, such as, for example, hair, bristles and feathers, or by enzymatic reaction of precursors.
For some years, L-cysteine has also been produced by fermentation using microorganisms that overproduce L-cysteine. WO 97/15673, EP-A-0885962 and WO 01/27307 describe processes for the production of O-acetylserine, L-cysteine or L-cysteine derivatives and related products using E. coli microorganisms.
WO 97/15673 relates to a process for the production of O-acetylserine, L-cysteine and sulfur-containing compounds derived therefrom, using feed-back resistant serine-acetyl transferases. Such serine-acetyl transferases have reduced sensitivity to the inhibitor L-cysteine in comparison with the wild-type enzyme.
EP-A-0885962 describes microorganisms containing at least one overexpressed gene coding for a protein that directly for the elimination from the cell of antibiotics or other substances toxic for the microorganism, and a process for the production by fermentation of L-cysteine, L-cystine, N-acetyl-serine or thiazolidine derivatives using such microorganisms.
WO 01/27307 describes a process for the production of L-cysteine or L-cysteine derivatives by means of fermentation of microorganisms, as well as microorganisms suitable for the process. Such microorganisms possess increased activity of the transcription regulator CysB, the CysB activity having a regulation pattern typical for a wild-type CysB.
L-Cysteine occupies a key position in the metabolism of sulfur and is used in the synthesis of proteins, glutathione, biotin, methionine and other sulfur-containing metabolites. However, it is also known that L-cysteine has reducing properties and accordingly has various problems in handling, such as, for example, stability.